Mechanisms of metabolism other than those mediated by the mixed function oxidases may be important in activating certain chemicals to their ultimate carcinogenic form. Recent evidence suggests that benzidine dyes are cleaved by intestinal bacteria, thereby liberating the parent benzidine and its congeners. There is evidence implicating peroxidases in the metabolism of benzidine dyes. Prostaglandin H synthetase is being used to activate compounds in the mouse lymphoma forward mutation assay. Modifications of the standard assay are necessary due to a high level of toxicity in this system, and because confounding reactions with the media during treatment have been observed. In addition, various substrates are being utilized. The possible mechanisms responsible for the formation of mutagenic metabolites induced by various peroxidase enzyme systems are being investigated. Selective inhibitors of the peroxidase are used to aid in the elucidation of these mechanisms. Studies include the identification of metabolites induced by this activation system.